unstabilized species; note that this is a small stabilization energy compared to the
resonance energy of benzene, most computational estimates of which are roughly
100 kJ mol"^1 (Section 5.5.2.2a). However, another experimental and computational
study of this question [ 60 ] led to the conclusion that triquinacene is not homoaro-
matic: combustion of the compound gave an enthalpy of formation ca. 17 kJ mol"^1
(4 kcal mol"^1 ) higher than that obtained from hydrogenation in [ 59 ] (241 vs 224 kJ
mol"^1 , 57.5 vs 53.6 kcal mol"^1 ). This negative conclusion was supported by
calculation of the heat of hydrogenation of a double bond in triquinacene and in
its di- and tetrahydro derivatives ( 1 , 2 , 3 , Fig.9.8), and by calculation of magnetic
properties of the triene and related molecules [ 60 ]. The heats of hydrogenation of
the double bonds were calculated with the aid of homodesmotic reactions, a kind
of isodesmic reaction (Section 5.5.2.2a) which preserves the number of each kind of
bond, and so in which correlation errors should cancel well; for 1 , 2 , and 3 the
calculated hydrogenation energy of a double bond are all essentially the same,
showing that a double bond of 1 is an ordinary cyclopentene double bond. Note that
using cyclopentane (Fig.9.8) rather than, say, ethane – which would also preserve
bond types – to (conceptually) hydrogenate 1 , 2 , and 3 should largely cancel out
energy differences due to ring strain. Interestingly, [ 59 ] concludes that “triquina-
cene is unequivocally stabilized” relative to reference species, but [ 60 ] asserts that,
from the thermochemical measurements, “The only logical conclusion is that
[triquinacene] is not homoaromatic.”
The magnetic properties used to probe aromaticity arise from the presence
of a diatropic ring current which tends to push an aromatic molecule out of a
magnetic field (calculated property: magnetic susceptibility,w), and which exerts
NMR shielding on a proton at or above the ring center (calculated property:
nucleus-independent chemical shift, NICS). NICS values are obtained from the
H(reaction) = – 2.5 kJ mol–1+3++12+2H(reaction) = –1.7 kJ mol–1+ + H(reaction) = +0.8 kJ mol3Fig. 9.8 The heat of hydrogenation of a double bond in triquinacene is essentially the same as that
of a double bond in dihdrotriquinacene and in tetrahydrotriquinacene, and is about the same as in
cyclopentene, indicating that triquinacene is not homoaromatic
9.1 From the Literature 571